1. Field of the Invention
The present invention relates to devices for capillary pressure and relative permeability testing of rock samples to evaluate petroleum deposits, and particularly to a spinning disc centrifuge rotor having a novel sample holder for performing spinning disc-type centrifugation of a cylindrical core sample.
2. Description of the Related Art
A centrifuge is a piece of laboratory equipment, generally driven by an electric motor (although some older models were spun by hand), which puts an object in rotation around a fixed axis, applying a force perpendicular to the axis. The centrifuge works through the sedimentation principle, where the centripetal acceleration causes more dense substances to separate out along the radial direction (i.e., toward the bottom of a collection tube). By the same principle, lighter objects will tend to move to the top of the collection tube (i.e., toward the center of rotation).
The centrifugal force acts as an effective gravitational force, and increasing this effective gravitational force will more rapidly and completely cause the precipitate (often referred to as the “pellet”) to gather on the bottom of the tube. The remaining solution is called the “supernate” or “supernatant”. The supernatant liquid is then typically either quickly decanted from the tube (without disturbing the precipitate), or withdrawn with a Pasteur pipette. The rate of centrifugation is specified by the acceleration applied to the sample, typically measured in revolutions per minute (RPM) or as a multiple of gravitational acceleration at the Earth's surface, g. The particles' settling velocity in centrifugation is a function of their size and shape, centrifugal acceleration, the volume fraction of solids present, the density difference between the particle and the liquid, and the viscosity.
Porous solids containing liquids provide unique challenges in centrifugation. For example, performing a capillary pressure measurement during centrifugation of a solid rock sample that contains liquid petroleum products requires a substantially different centrifugation procedure than performing centrifugation on liquid solution or a mostly liquid suspension.
FIG. 2 diagrammatically illustrates an early centrifuge system 100 for studying capillary pressure and similar properties in a core sample S, which may be a core sample of rock containing petroleum products or the like. In the system 100, the core sample S is fixed at one end to a rotating arm 102, which is, in turn, fixed at its other end to a rotating axle A. As the axle A spins, as in conventional centrifugation, the liquid petroleum products within sample S migrate through the porous rock, from the inner radius R1 toward the outer radius R2.
This relatively simple arrangement, however, makes both direct measurements and calculated predictions extremely difficult. In order to properly calculate capillary pressure, for example, a model must be used in which the rock sample is assumed to be homogeneous, the sample S must be perfectly cylindrical, and centrifugal acceleration must be parallel to the cylindrical axis of the sample S at all times.
In order to make measurements easier, and also to provide for greater accuracy in calculations, an arrangement for centrifugation as shown in FIG. 3 is desirable. As shown, the cylindrical core sample in FIG. 3 is rotated about a central axis X, which is coaxial with the cylindrical axis of the sample itself. FIGS. 4 and 5 illustrate a simplified prior art “spinning disc” centrifuge 200 for centrifuging a cylindrical core sample about the cylindrical axis, as in FIG. 3.
In the centrifuge 200, a sample cell 206 is mounted on a plate 202. As shown in FIG. 4, the sample cell 206 has a cylindrical sample chamber 212 formed therein for receiving a cylindrical core sample. The plate 202 may be mounted on a rotating support 204, which is coupled (via a conventional coupler 208) to a driven rotating axle 210. A plurality of collection vessels 214 are in communication with the sample chamber 212 (extending radially outwardly therefrom) to collect liquids that flow from the porous rock sample under centrifugal force.
The spinning disc centrifuge 200 provides a major improvement over the system 100, allowing for the testing of large core samples, providing validity for the assumption of zero capillary pressure at an outlet face, allowing for the measurement of local saturation at any given time, and providing the possibility of estimating drainage and spontaneous capillary pressure. These, along with consideration of radial effects and the ability to estimate relative permeability using both fluid production and saturation distribution, however, are all only within the realm of possibility with the conventional spinning disc system 200. Thus far, no spinning disc centrifuge has been produced that effectively allows for such complete measurement and testing.
Further, although such an arrangement is an improvement over the simple system 100 shown in FIG. 2, the centrifuge 200 does not allow for consolidated samples. Typically, only an unconsolidated sample could be used in system 200, with a plurality of mesh screens being provided between the sample chamber 212 and the collection vessels 214. Further, performing pressure measurements remains extremely difficult in this arrangement, since the collection vessels 214 are best used solely for collection following the completed centrifugation process.
In addition to the above, it would be desirable to be able to easily study the definition of boundary condition on the end faces of a cylindrical core plug, the gravity degradation effect at low speeds, the characterization of core sample heterogeneity, and the effect of radial centrifugal field distribution inside a core sample. Particularly, a core sample holding rotor allowing for the accurate measurement of capillary pressure and relative permeability curves would be desirable.
Thus, a spinning disc centrifuge rotor solving the aforementioned problems is desired.